A chemical amplification resist composition is a pattern forming material of forming a pattern on a substrate by producing an acid in the exposed area upon irradiation with an actinic ray or radiation such as far ultraviolet light and through a reaction using the acid as a catalyst, changing the developer solubility of the area irradiated with an actinic ray or radiation and that of the non-irradiated area.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between a projection lens and a sample has been conventionally known as a technique for enhancing the resolution in an optical microscope.
As for the “effect of immersion”, assuming that NA0=sin θ, the resolution and depth of focus in immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, when the projection optical system has the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and furthermore, can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
Examples of the apparatus where the effect above is applied to transfer of a fine image pattern of a semiconductor device is described in, for example, Patent Documents 1 and 2.
Recent technical progress in the immersion exposure is reported, for example, in Non-Patent Document 1 and Patent Document 3. In the case of using an ArF excimer laser as a light source, pure water (refractive index at 193 nm: 1.44) is considered to be most promising as the immersion liquid in view of safety in handling as well as transmittance and refractive index at 193 nm. In the case of using an F, excimer laser as a light source, a fluorine-containing solution is being studied from the aspect of balance between transmittance and refractive index at 157 nm, but a solution satisfied in terms of environmental safety and refractive index has not yet been found. Considering the degree of immersion effect and the perfection of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Also, it is pointed out that when the chemical amplification resist is applied to immersion exposure, the resist layer comes into contact with the immersion liquid at the exposure, as a result, the resist layer deteriorates or a component adversely affecting the immersion liquid bleeds out from the resist layer. Patent Document 4 describes a case where when a resist for ArF exposure is dipped in water before and after exposure, the resist performance is changed, and this is indicated as a problem in the immersion exposure.
As regards the medium filled between a projection lens and a semiconductor substrate, which is used in the immersion exposure, water having a refractive index of 1.44 is employed in view of easy availability and safety and by using an exposure machine having a projection lens with NA of 1.2 to 1.35, pattern formation of a semiconductor device in a design dimension up to a 45 nm generation is considered to be possible.
The generation next to the design dimension of 45 nm is a 32 nm generation, and it is considered that NA of 1.65 is necessary for the pattern formation of a 32 nm-generation semiconductor device and in this case, the medium filled between a projection lens and a semiconductor substrate must have a refractive index of 1.8 or more.
Meanwhile, the material of a projection lens having NA of 1.65 is required to have a refractive index of 1.9 or more, and LuAg is currently supposed to be a promising candidate therefor, but its problem of absorbing a large amount of light passed has not yet been solved.
Furthermore, a candidate medium having a refractive index of 1.8 or more has also not yet been found.
For these reasons, there is attracting attention a method where a special pattern forming method using an exposure machine with a projection lens having NA of 1.2 to 1.35 is used for the pattern formation of a 32 nm-generation semiconductor device.
Several methods have been proposed for this special pattern forming method, and one of these methods is a double exposure process.
The double exposure process is, as described in Patent Document 5, a process of exposing the same photoresist film two times, and this is a method where the pattern in the exposure field is divided into two pattern groups and the exposure is preformed in twice for respective pattern groups divided.
Patent Document 5 indicates that this method inevitably requires a property like a two-photon absorption resist, that is, a property of the photosensitivity or developer solubility being changed in proportion to the square of exposure intensity, but a resist having such a property has not yet been developed.
Also, a compound capable of decomposing by the action of an acid to generate an acid is described in Non-Patent Documents 2 and 3.
On the other hand, in respect of the pattern forming method for a 32-nm generation semiconductor device, development of lithography using electron beam, X-ray or UV light is also proceeding.
In particular, the electron beam lithography is positioned as a next-generation or next-next-generation pattern formation technique, and a positive resist assured of high sensitivity and high resolution is demanded. Above all, the elevation of sensitivity is very important so as to shorten the wafer processing time, but in the positive resist for electron beam, when higher sensitivity is sought for, this incurs not only reduction in the resolution but also worsening of the line edge roughness, and development of a resist satisfying all of these properties at the same time is strongly desired. The line edge roughness as used herein means that the resist edge at the interface between the pattern and the substrate irregularly fluctuates in the direction perpendicular to the line direction due to resist characteristics and when the pattern is viewed from right above, the edge gives an uneven appearance. This unevenness is transferred in the etching step using the resist as a mask and gives rise to deterioration of electrical properties and in turn, reduction in the yield. Particularly, in the ultrafine region of 0.25 μm or less, the line edge roughness is a very important problem to be solved. The high sensitivity is in a trade-off relationship with high resolution, good pattern profile and good line edge roughness and it is very important how to satisfy these properties all at the same time.
In the case of using an EUV light source, the light belongs to an extreme-ultraviolet wavelength region and has high energy, and a concerted photochemical reaction such as negative conversion ascribable to EUV light is brought about, which gives rise to a problem such as reduction in the contrast. Accordingly, it is an important task also in the lithography using X-ray or EUV light to satisfy all of high sensitivity, high resolution and the like, and this task needs to be solved.
As regards the resist suitable for such a lithography process using electron beam, X-ray or EUV light, a chemical amplification resist utilizing an acid catalytic reaction is mainly employed from the standpoint of elevating the sensitivity, and in the case of a positive resist, a chemical amplification resist composition containing, as main components, an acid generator and a phenolic resin having a property of being insoluble or sparingly soluble in an alkali developer and becoming soluble in an alkali developer by the action of an acid (hereinafter simply referred to as a “phenolic acid-decomposable resin”) is effectively used.
With respect to such a positive resist for electron beam, X-ray or EUV light, as described, for example, in Patent Documents 6 to 11, some resist compositions containing a phenolic acid-decomposable resin have been heretofore known.
Also, as described in Patent Documents 12 and 13, a radiation-sensitive resin composition having blended therein a compound capable of decomposing by the action of an acid to generate an acid is known.
However, these techniques have not succeeded so far by any combination in satisfying all of high sensitivity, high resolution, good pattern profile and good line edge roughness in the ultrafine region.
Patent Document 1: JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 2: JP-A-7-220990
Patent Document 3: International Publication No. 04/077158, pamphlet
Patent Document 4: International Publication No. 04/068242, pamphlet
Patent Document 5: JP-A-2002-75857
Patent Document 6: JP-A-2002-323768
Patent Document 7: JP-A-6-41221
Patent Document 8: Japanese Patent No. 3,173,368
Patent Document 9: JP-A-2000-122291
Patent Document 10: JP-A-2001-114825
Patent Document 11: JP-A-2001-206917
Patent Document 12: JP-A-2000-35665
Patent Document 13: JP-A-2007-114431
Non-Patent Document 1: Proc. SPIE, Vol. 4688, page 11 (2002)
Non-Patent Document 2: Chem. Lett., 1036 (2000)
Non-Patent Document 3: The Chemical Record, Vol. 2, 46-55 (2002)